Known in the state of the art are numerous membranes in which the material-selective separation layer is made of polyvinyl alcohol. A chemical stability of these membranes and of the material-selective separation layers, for example with respect to water, is thereby achieved in that the polyvinyl alcohol is cross-linked. The known membranes of this kind are suited principally for use in the separation of vaporous media or of gas-vapor mixtures up to temperatures of 105° C. At temperature exceeding 105° C. stability problems occur with such membranes. Such a membrane, wherein the material-selective separation layer is made of cross-linked polyvinyl alcohol is described in DE-A-32 20 570.
However other membranes based on polyvinyl alcohol are known, for example from DE-C-40 04 153, wherein a membrane is described having an ordered structure with a large microcrystalline portion, with the crystallization having been achieved by holding the solution at temperatures from −10 to +10° C. for several days.
For this membrane also the guaranteed stability of the membrane does not hold for temperatures higher than 105° C.
In summary it can be stated that all known membranes whose material-selective separation layer is made on the basis of polyvinyl alcohol at temperatures lying above 105° C. do not exhibit sufficient mechanical and chemical stability for the dewatering of organic solvents by means of vapor permeation and also have an unsatisfactory stability with regard to the formation of a condensate on the outer surface of the membrane. Because of their unsatisfactory mechanical stability at temperatures above 105° C. they also have the disadvantage that they are not suited to being drawn upon for the making of padded membranes (membrane pads, membrane pockets), since such membrane elements for their making have to be welded at their edge regions, which with previous membranes based on a material-selective separation layer of polyvinyl alcohol is not possible.